The present invention relates to a method of and an apparatus for printing serial numbers on corners of sheets of value tokens such as bank notes, securities and the like while the value tokens are being printed on the sheets.
It is of prime importance in the printing of value tokens such as bank notes, securities, postage stamps and the like that a record be made of how many value tokens have been printed. Printing presses used for printing such value tokens are generally equipped with counters for recording the number of revolutions of the press and the number of printed sheets that have passed through the press. The printing press also has a serial number printing unit for printing a serial number 2 (FIG. 1 of the accompanying drawings) on a corner of a trimming margin of each printed sheet 1. The printed serial number allows an easy and quick determination of how many sheets have been printed during or at the end of the printing process.
FIG. 2 of the accompanying drawings is a schematic side elevational view of a conventional printing press, showing a serial number printing unit and adjacent parts. A sheet 4 fed, one at a time, onto a feedboard 3 from a paper feeder is transported as it is successively gripped by a swing pre-gripper 5 and grippers on transfer cylinders 6, 7 and an impression cylinder 8 which are rotated in the directions of the arrows. A desired pattern or image is printed on the sheet 4 between the impression cylinder 8 and a plate or blanket cylinder (not shown). The serial number printing unit, generally designated by the reference numeral 9, has a numbering device 10 rotatable about its axis in the direction of the arrow for periodical contact with the sheet 4 travelling around the transfer cylinder 7. The numbering device 10 is constructed such that its number, which is to be printed on the sheet 4, will increase in a single increment each time the numbering device 10 makes one revolution. Above the numbering device 10, there is provided an inking device 18 comprising an ink fountain 11 for containing ink for printing serial numbers, an ink fountain roller 12, a ductor roller 13, a vibrator roller 14, a transfer roller 15, a vibrator roller 16, and an ink form roller 17. A film of ink which is supplied from the inking device 18 to the printing surface of the numbering device 10 is transferred onto a corner of the sheet 4 as it is gripped by the grippers on the transfer cylinder 7 and directed toward the impression cylinder 8, whereupon a serial number is printed on the sheet 4.
With the prior serial number printing unit 9, the serial number is printed by the numbering device 10 on the surface of the sheet 4 which will face a blanket 19 on the periphery of the impression cylinder 8 when the sheet 4 is wound on the latter. Therefore, the printed serial number tends to be transferred from the sheet 4 onto the blanket 19 under pressure imposed when a pattern is printed on the sheet 4. The transferred serial number is then transferred from the blanket 19 back onto a next sheet 4 where it bears a newly printed serial number. This difficulty has been eliminated by cutting off the blanket 19 at a position aligned with printed serial numbers to avoid the transfer of the latter from the sheets 4 onto the blanket 19. Such a recessed blanket, however, is disadvantageous in that it tends to be placed in a distorted disposition with respect to the impression and to be broken from the recess, resulting in a shorter service life of the blanket 19, and a sharp pattern or image cannot be printed on the sheets 4.